Supermarket lights help keep spinach fresh and producing new vitamins, according to US government researchers.

The surprising findings should apply to other fresh vegetables and may offer insights into how to keep produce fresher longer, the researchers reported in the Journal of Agricultural and Food Chemistry.

Supermarkets often display fresh spinach in clear plastic containers at around 4°C under fluorescent light 24 hours a day. Lester wondered if this was good or bad for the leaves.

"It is about time we asked some of these questions and do some of the science," says Lester.

Stay fresh

His team kept fresh spinach leaves under continuous light or darkness for three to nine days.

Spinach kept under lights for as little as three days had significantly higher levels of vitamins C, K, E and folate, as well as more the colourful and healthful carotenoids lutein and zeaxanthin, they report.

Leaves stored in the dark lost nutrients, says Lester. On reflection, he says, the findings should not be surprising.

"These vitamins are basically in the plant for photosynthesis and we humans, being the biggest predator of plants, have evolved over time to utilise them as opposed to we having to manufacture them," he says.

Even when picked, leafy greens continue to photosynthesis, says Lester. "As long as there is moisture in the leaves and as long as there is gas exchange and light, it is good to go whether they are picked or not."

His team chose fresh spinach as it is "arguably one of the most nutritionally complete vegetables commonly consumed." A serving of spinach provides 20% or more of the recommended dietary intake of vitamins C, A, B9, K and E.